EVALUATION OF MODEL BEHAVIOR WITH RESPECT TO THE BIOGEOCHEMISTRY AT THE SOLLING SPRUCE SITE

The performance and predictions of 11 biogeochemical models, applied t o time series data from a spruce site in Solling, Germany were evaluat ed. All models are deterministic, process-oriented models, and represe nt a wide range of modelling approaches with respect to time and space resolution and complexity. Although process formulations vary, the ba sic processes are shared by most models. The scope of the models is to calculate soil chemistry characteristics relevant to assess stress fa ctors for forests (e.g. unfavourable high inorganic Al concentrations and Al/Ca rations in soil solution) induced by acid deposition. The ev aluation showed that the general trends and levels in chemical variabl es, such as soil solution pH, and concentrations of base cations, SO42 - and Al, as well as base saturation can be reproduced by the models. Most models are incapable of modelling correctly pH and Al concentrati ons simultaneously. Modelling NO3- concentrations was generally not ve ry successful, despite several complex modelling approaches. The model ling efforts highlighted that the concentrations of base cation, SO42- and Al-i are coupled in a complex and nonlinear way, which has strong implications for the response to reductions in acid deposition. The r ate of chemical weathering obtained by budget studies was supported by the models. The comparison of the models showed that the links betwee n soil chemistry and to forest growth are incomplete. It is recommende d to continue to work on certain processes such as Al-chemistry and N transformations, and on the interaction between soil chemical status a nd biota.